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8 Answers
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Its difficult to give a reliable source for this, but searching in google it appears these words, for example in this page. But it's difficult to say this because there are lots of technical terms which are difficult to list, because many of them can be formed by adding a lot of prefixes.

Superextraordinarísimo, (22-letters) was once listed as the word most often cited to be the longest in spanish. (It means "most superextraordinary"), however superextraordinarísimo seems arbitrary at best. For one thing, the word isn't in real use. As of this writing, a Google search shows not a single instance where the word was used on a Spanish-language Web site — except on pages listing what they called the longest Spanish words. And superextraordinarísimo has two other strikes against it: If one is going to create words by adding prefixes and suffixes, one could just as well make a 27-letter word by using the adverbial form, superextraordinarísimamente. Or one could just as easily use longer root words, ending up with words such as superespectacularísimamente ("most superspectacularly"). But again they're hypothetical words rather than ones that get legitimate use.

Also, the longest English word listed in an accepted dictionary is reported to be pneumonoultramicroscopicsilicovolcanokoniosis, a type of lung disease. Conversion of the word to Spanish, which is made easy by the fact that all the roots have Spanish cognates, presumably would be neumonoultramicroscopicosiliciovolcanconiosis (45 letters), or something similar. But such words would probably be considered spurious rather than legitimate Spanish.